Wise Intervention
Direct (DI) boxes are often a key in this quest. Not only do they convert the 1/4-inch phone plug output of a keyboard or guitar to XLR, they also drop the level about 22 dB.
So if we take the line level output of a keyboard (maybe +4 dBu) and attenuate it by 22 dB, the result will be a mic level signal of -18 dBu, because +4 dBu minus 40 dB equals -18 dBu (see how that works?).
DIs (Figure 5) incorporate transformers that do other good things as well, such as letting us lift the chassis grounds between stage gear and consoles to eliminate ground loop hum, as well blocking 48-volt phantom power from back-feeding into gear and blowing up the outputs. This is why we really don’t want to use a 1/4-inch phone to XLR male cable to plug in stage gear. The signal level may be too hot for a mic level input on a console, plus there’s no ability to do a ground lift to stop hum.
But the real danger is when this cable is (accidentally) plugged into the loudspeaker output of a stage amp and then connected to the snake feeding a console’s XLR inputs. Recall the stage gear with both preamp and loudspeaker outputs on 1/4-inch jacks? Here’s a story that should make us think twice about utilizing adapter cables to connect a guitar or bass amp to a console instead of using a DI.
Solving The Mystery
Over the past 20 years or so, I’ve taught audio seminars at hundreds of churches, and occasionally, I come across a console with a fader or two taped over with the universal sign of a big “X” to indicate a dead channel. I ask the tech folks if they know what happened, and the universal answer is usually something like “lightning must have hit the building and burned out the channel (s).” However, the fact is that if lightning actually did hit the facility’s electrical service, it would burn out more than just a channel or two on a console.
I’d never received a better explanation until a visit to a church with a console with three faders in a row marked as dead. I asked what had happened, and they indeed knew the circumstances: there was a long-standing habit of using a 1/4-inch to XLR male adapter cable to connect the preamp output of a guitar amp to the main PA system. Because it was only sending +4 dBu line level to the console, and the preamp was OK with +4 dBu (which any good preamp should be), it worked fine for years.